On Wed, 01 Jul 2015 17:23:54 -0400, Jerry Stuckle
wrote:
Jeff, do you always miss the forest for the trees? That was an EXAMPLE.
Perhaps you missed my point. I don't care about VSWR as long as the
system is reasonably efficient, does not protect itself, and produces
adequate TX power. In my world, that means I'm concerned about
losses, not VSWR. The losses involved in using 75 ohm feedline and
coax in a 50 ohm system are negligible.
The same would be true if you were feeding a 300 ohm yagi with 300 ohm
twinlead and a transmitter with a 10 ohm output impedance.
One of my favorite methods of argumentation is to provide a ridiculous
and extreme example, and then use it as the basis for discussion. I
guess it's a form of "straw man" argument, where I'm now expected to
defend my point of view against your ridiculous and extreme example.
Please forgive me for not following your lead and continuing to be
more concerned with the 50/75 ohm problem:
https://en.wikipedia.org/wiki/Straw_man
And BTW - when calculating, you forgot about the transmitters which cut
back power to protect the finals. Many will do so even with a 1.5:1 SWR.
Could you provide me with one example of such a transmitter? I've
seen such radios on the bench, but they're usually mistuned or
misadjusted.
I did some Googling and found that the typical threshold for both AM
and FM broadcast xmitters is 1.5:1.
https://www.google.com/#q=transmitter+high+vswr+threshold+1.5:1
Same with most HF radios that I could find. The stuff I designed for
marine use (150 watts PEP) was set to operate up to 2.0:1 because at
the time, ATU's were just appearing and the typical vessel HF antenna
was problematic (23ft vertical with a dubious ground system). Today,
1.5:1 threshold would probably work.
Note that such a threshold does NOT mean that at 1.4999:1, the radio
would work normally, and at 1.5001:1 would shut down. For (marine)
radios that are expected to work with random antennas in emergencies,
shutting down at 1.5:1 is absurd. What is normally done is to slowly
reduce the drive starting at 1.5:1 until it gets to some point below
where either the final current is too high to maintain safe
dissipation, or the voltage across the final xsistor or FET is too
high to prevent breakdown. My guess(tm) is that's about 5:1 or more
with todays radios but I'll admit that I'm guessing and haven't
actually tried it. In the past, I used a test load, that someone else
built, that would provide a resistive 2:1 VSWR at 25/37.5/50/75/100
ohms, and an adjustable phase angle with a big variable capacitor and
roller inductor. I had to be VERY careful not to accidentally tune
the inductor and capacitor to resonance, or I would end up with a
short or open load. The dials had an accompanying chart that followed
varioius constant VSWR circles around the Smith chart.
The tricky part was not making the power amp work over a 4:1 impedance
range. The tricky part was making a VSWR sensor that would be fairly
flat over the entire 2 to 30 MHz range. Another headache was when a
mismatch caused the PA to draw more current. It wasn't final heating
that caused instabilities and odd behavior. It was my worthless bench
power supply that would detect the overcurrent and protect itself by
dropping the output voltage.
So, which radio shuts down at less than 1.5:1 VSWR? I know of a few
possible candidates, but I would like to see what you've observed
first.
--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060
http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558